Some internal combustion engines include a positive crankcase ventilation (PCV) system to route gas (i.e., PCV gas) in a crankcase to an intake manifold of the engines. Some engines further include a water-cooled charge air cooler (WCAC) positioned in an intake manifold. The WCAC'sfins and plate can act as a separator to separate oil droplets from inlet air and oil droplets from PCV gas. The separated oil accumulates at a bottom of an intake manifold and would reach a critical amount. When the accumulated oil exceeds the critical amount, the oil may ingress a combustion chamber when a driver operates a vehicle at a deep acceleration. The inventors of the present application have recognized that there is need for an intake manifold drain system to drain and recirculate the oil accumulated in the intake manifold.